This invention concerns a hologram recording medium and, particularly, relates to a volume phase hologram recording medium.
The material used for recording volume phase holograms includes dichromated gelatines (hereinafter referred to as DCG), bleached silver halide photographic emulsions, photosensitive polymers and inorganic dielectric materials such as LiNbO.sub.3.
Recording mediums using organic polymeric materials generally provide high varying refractive indices and photo-induced cross-linking reactions or photo-induced change of the materials also occurs, which results in dimensional instability or large shrink-expansion stresses that cause distortion in the interference pattern. Accordingly, there is a drawback that such organic polymeric materials are not suitable for use where a high accuracy interfernece pattern is required.
On the other hand, inorganic dielectric materials have a drawback that the varying refractive indices are lower as compared with those of the organic materials, although their dimensional stability is excellent.
In order to overcome the foregoing drawbacks, there has been proposed in U.S. Pat. No. 4,187,111, a composite system hologram comprising a porous glass material and a highly polymeric monomer. In the proposed hologram, pores of a porous substrate are filled with a photopolymerization initiator and an image exposed, that is, a holographic exposure of a predetermined pattern to form a pattern composed of a portion forming radicals and a portion not forming them, followed by developing treatment. In the developing treatment, the pores of the substrate are filled with a monomer and, thereafter, the monomer in the pores is selectively polymerized by the entire exposure at the above-mentioned radical-forming pattern, thereby preparing a varying refractive index of a required pattern, that is, an interference lattice.
In this hologram, however, the adhesion strength of the photopolymerizable monomer or the photopolymerization initiator to the pores of the porous substrate cannot be increased, aging degradation has resulted in the varying refractive index after the formation of the hologram.
It is the object of this invention to provide a hologram recording medium which uses a composite system comprising a porous material and a photopolymerizable monomer, which has excellent dimensional stability, of a relatively large varying refractive index and capable of overcoming the aging degradation.
These and other objects of this invention will become apparent from the following description and appended claims.